1. Field of the Invention
This invention relates to a glazing unit alarm system of the kind comprising an electric alarm circuit and a glazing panel having a light transmitting electrically conductive coating extending across a major face of the panel and forming part of the alarm circuit, the alarm circuit being arranged to be activated on detection of a change in resistance of the electrically conductive coating occasioned by a breakage or an attempted breakage of the glazing panel. The invention is primarily, although not exclusively, intended for application in double glazing units.
2. Description of Related Art
Known glazing unit alarm systems of the kind referred to have previously been disclosed in GB-A-No. 2026219, GB-A-No. 1256641 and U.S. Pat. No. 3,825,918. For example in GB-A-No. 1256641 a chemical deposition/etching technique is disclosed for producing a sinuous pattern for the electrically conductive coating. In U.S. Pat. No. 3,825,918 a metallized layer is applied to a backing film which is subsequently applied to the glass panel. In GB-A-No. 2026219 various examples are given of electrically conductive strips being applied in a precise pattern or of a continuous electrically conductive coating being formed by a thermal decomposition process. However, in each of these known glazing unit alarm systems, an electrically conductive strip or coating is incorporated into a glazing unit for the sole purpose of providing a conductive path as part of an alarm circuit. The provision of these metallized layers greatly increases the complexity of, and cost of producing, such glazing units and heretofore has led to such glazing unit alarm systems of the kind referred to only normally being provided for high security windows where the cost of the window is of secondary importance to its security aspect.